A conventional seat adjusting mechanism of an exercise machine, as shown in FIGS. 6 and 7, comprises a base frame 70 and a seat 73 that can slide on the base frame 70. A positioning mechanism 7 is provided in-between the seat 73 and the base frame 70 for positioning the seat within the pre-determined location of its moving range, which comprises a gear rack 71 fixed on the base frame 70 on which a plurality of dents 72 are provided therealong, a fit-in block 74 that pushed by a spring 77 and has a plurality concavities 78 for the dents 72 of the gear rack 71 to match with, and a lever 75 that is pivotally installed on the seat 73. One end of the lever 75 is connected to the fit-in block 74 and the other end of the lever 75 has a levering handle 76 for users to activate the mechanism. The conventional structure by activating the lever 75 for having the concavities 78 of the fit-in block 74 fitted, or not fitted, with the dents 72 of the gear rack 71. When the concavities 78 are fitted with dents 72, the seat 73 is fixed with the base frame 70 and cannot be moved. When the concavities 78 are not fitted with dents 72, users are able to adjust the location of the seat 73 relatively to the base frame 70. Because gear rack is of high price so the cost is high, besides, there is a certain distance between two dents such that adjustment tuning cannot be effectively achieved. Furthermore, the concavities and the dents are easy to get damaged because they contact with each other during fitting-in.